1. Field of the Invention
The present invention relates to a technique for setting an inspection standard to be used in process inspection in an intermediate process on a production line.
2. Description of the Related Art
On a production line comprising a plurality of processes, process inspection for determining pass or fail of an intermediate product is performed at each intermediate process as well as a final inspection for determining overall pass of fail of a product.
The goal of the process inspection is to find an intermediate product that does not satisfy a quality level determined during the process and to prevent the defective product from entering the next process. In order to do this, an inspection standard (for example a threshold value for determining pass or fail of the intermediate product) is set according to a quality level (tolerance, etc.) that is determined in each process.
However, an intermediate product which satisfies a certain quality level does not always become a good product at the last stage. Even when the intermediate product satisfies the quality level, it could contain a potential defect factor which induces a defect in the following process. Such a potential defect factor is referred to as a “defect sign” hereinafter. Thus it is desirable to reject not only the intermediate product that does not satisfy the quality leave, but also an intermediate product that has a defect sign, preventing the defect from being generated in the following processes.
Nonetheless, it is very difficult to appropriately set an inspection standard to detect the defect sign because it is difficult to clarify and quantify a causal correlation between the degree of the defect sign and probability of defect generation in the following process. Furthermore, when there are several inspection items in the process inspection, it is difficult to specify which inspection standard of the inspection items was effectively adjusted. If the inspection standard is too strict because of a high priority on finding a defect sign, the yield rate (the rate of the number of the accepted products to the total number in the inspection) is lowered, causing the yield to deteriorate or reinspection costs to increase.
A technique to support inspection standard setting is disclosed in Japanese Unexamined Patent Publication No. 2004-214394 and is well known. However, according to this method, although an over detection rate of the inspection result is calculated and displayed for a user to confirm whether or not the over detection rate satisfies a predetermined management standard, it only urges the user to determine whether it is necessary to adjust an inspection standard. That is, this method is cannot be used for determining how the inspection standard of an inspection item of a certain process is adjusted.